Method for preserving electron gun assemblies

ABSTRACT

A method for preserving electron gun assemblies is disclosed. In the method, an electron gun assembly, including a thermionic cathode emitter, is assembled in a vacuum envelope structure. The envelope is evacuated, baked out, and the cathode processed in the conventional manner to activate the cathode. Beam voltages are applied to draw beam current from the gun to test proper operation thereof. These tests may include a test of beam diameter and permeance in the presence of a beam focusing magnetic or electric field. The processed and tested electron gun is then valved off from the rest of the envelope by means of a valve connected to the envelope portion including the electron gun. The valved-off gun is then detached from the vacuum envelope for preserving the electron gun in an operable condition for subsequent use in a second vacuum envelope structure.

United States Patent (72] Inventors George V. Miram Daly City; Samuel W.Woolsey, Los Altos, both of, Calif. [2]] Appl. No. 803,774 [22] FiledMar. 3, 1969 [45] Patented June 1S, 1971 [73] Assignee Varian AssociatesPalo Alto, Calif.

[54] METHOD FOR PRESERVING ELECTRON GUN ASSEMBLIES 4 Claims, 2 DrawingFigs.

[52] U.S.Cl 316/1, 316/24, 316/26 [51] lnt.Cl H0lj 9/00 [50] Field ofSearch 316/1, l7, l8, i9, 24, 26

[56] References Cited UNITED STATES PATENTS 3,366,434 1/1968 Mengel316/1 Primary Examiner-John F. Campbell Assistant Examiner-RichardBernard Lazarus Attorneys-Stanley Z. Cole and Leon F. Herbert ABSTRACT:A method for preserving electron gun assemblies is disclosed. In themethod, an electron gun assembly, including a thermionic cathodeemitter, is assembled in a vacuum envelope structure. The envelope isevacuated, baked out, and the cathode processed in the conventionalmanner to activate the cathode. Beam voltages are applied to draw beamcurrent from the gun to test proper operation thereof. These tests mayinclude a test of beam diameter and permeance in the presence of a beamfocusing magnetic or electric field. The processed and tested electrongun is then valved off from the rest of the envelope by means of a valveconnected to the envelope portion including the electron gun. Thevalved-off gun is then detached from the vacuum envelope for preservingthe electron gun in an operable condition for subsequent use in a secondvacuum envelope structure.

PATENTEUJUNISIHYI 3.585445 LYSTRON KLYSTRON KLYSTRON FIG.2

UNCHER LERATOR ACCELERATOR ACCE ERAIOR INVENTORS GEORGE V.M|RAM 33SAMUEL WWOOLSEY BY 3%, MM;

ATTORNEY KLYSTRON METHOD FOR PRESERVING ELECTRON GUN ASSEMBLIES Theinvention described herein was made in the course of, or underSubcontract 5603 under Contract W-7405 Eng -26 with the Atomic EnergyCommission.

DESCRIPTION OF THE PRIOR ART Heretofore, a valve has been providedbetween the electron gun portion of a beam tester and thebeam-collecting portion of the beam tester for valving off the electrongun to prevent contamination of the gun during periods wherein thecollector portion of the beam tester may be exposed to atmosphericpressure. Such an arrangement is described in Research Report EE-495 ofCornell University School of Electrical Engineering, dated 1 May l96l,further identified as RADC-TN-l-l 88, Linear Beam Microwave TubeTechnical Report No. 9, under Contract No. AF30(602)-l696. It was foundin this prior work that the provision of a ball valve between theelectron gun and, the remaining portion of the beam tester permitted theoperator to valve the electron gun off from the remaining portion of thevacuum envelope of the beam tester during times when the beam test'erwas not in use to prevent contamination-of the electron gun bybackstreaming of oil from the diffusion pumps connected to the main beamtester envelope section. A separate vacuum pump was connected to theelectron gun assembly fol" maintaining the vacuum in that portion ofvacuum envelope containing the gun when the gun was valved-off from theremaining portion of the envelope.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of a method for preserving electron gunassemblies.

One feature of the present invention is the provision of a valved-offvacuum envelope containing a processed electron gun assembly forsubsequent use in a second vacuum envelope structure, whereby theoperating parameters of the electron gun can be guaranteed and wherebysubstantial shelf life can be obtained.

Another feature of the present invention is the same as the precedingfeature including a glow discharge getter ion vacuum pump connected tothe valved-off electron gun assembly for maintaining a low pressure andfor periodically monitoring the gas pressure within the valved-offelectron gun.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the valved-off electron gun assemblyis connected by a suitable flange structure to a second envelope, suchas a particle accelerator, which, is to use the gun when the valve isopened for placing the gun in gas communication with the second vacuumenvelope.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a longitudinal view, partlyin section and partly schematic, of a portion of an electron gun testerincorporating features of the present invention, and

FIG. 2 is a longitudinal view, partly in section, of a particleaccelerator incorporating the detachable electron gun assembly of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. I, thereis shown a beam tester apparatus l, for practicing the method of thepresent invention. The gun tester 1 includes a typical electron gunassembly 2 having a spherical concave thermionic cathode emitter 3, asof oxide coated nickel, impregnated tungsten, or other conventionalemitter material, which is heated to operating temperature via heater 4to produce a stream of electrons 5 passable through the central aperture6 in an anode structure 7. A cylindrical high voltage insulatorstructure 8 forms a portion of the vacuum envelope for the gun 2, issealed at one end to the anode 7, and includes suitable feedthroughconnector assemblies at the opposite end thereof, not shown, forapplying the operating potentials to the elements within the gun. A gridstructure 9, having a concave surface conforming to the radius ofcurvature of the cathode emitter 3, is disposed in front of the emitter3 for controlling the beam current.

The anode electrode 7 includes an annular disc portion 11 made of amagnetic permeable material such as magnetic stainless steel to form theanode pole piece of a magnetic beam focusing structure. A conductiveinsert 12, as of copper, is inserted into the aperture in the center ofthe pole piece 11 to form the proper shape of the electrostatic field ithe region between the cathode and the anode while permittiiig the polepiece aperture to be shaped to produce the proper magnetic field shapingthrough the anode.

A magnetically confined glow discharge getter ion vacuum pump 13 isaffixed to the downstream side of the anode pole piece 11 via a circularpipe-coupling flange 14. The vacuum pump 13 includes a tubular main body15 disposed in axial alignment with the beam path for passage of thebeam I therethrough. A pumping chamber 16 surrounds the tube 15 andcontains therewithin a plurality of Penning anode cells disposed betweena pair of cathode plates of getter material. Pumps of this general typeare described in U.S. Pat. No. 3,125,283. A horseshoe-shaped magnet 17,shown in phantom lines, is disposed with its north and south polestraddling the pumping chamber 16 to produce an axially directedmagnetic field through the anode glow discharge passageways therein forconfining the glow discharge therewithin. Although only one magnet 17appears in FIG. 1, a plurality of such magnets are normally spacedaround the pump chamber as shown in said U.S. Pat. No. 3,125,283. Themagnet 17 is not required when the beam 5 is magnetically focused bymeans of a magnetic beam focus solenoid 18 shown in dotted lines.Alternatively, the beam may be focused by periodic electrostatic ormagnetic lenses in which case the pump 13 will require its own magnet17.

A bakeable straight through high vacuum valve 19, such as VarianAssociates Model No. 951-5052, is affixed to the vacuum pump 13 via apair of mating bakeable high vacuum flanges 20 and 21 affixed to thevacuum pump and straight through valve, respectively. Thestraight-through valve 19 includes a straight-through tubular bodyportion 22 coaxially aligned with the beam path 5 for passage of thebeam through the valve when the valve 19 is in the open position. Theother end of the valve 19 is sealed, in a vacuum-tight manner, to adisc-shaped collector pole piece 23, as of soft iron, via theintermediary of a demountable high-vacuum flange assembly 24. The flangeassembly 24 includes a first flange member 25 carried at the end of thevalve body 22. Flange 25 mates with a flange member 26 sealed to thecollector pole piece 23. The collector pole piece 23 is centrallyapertured at 27 for passage of the beam therethrough into a hollowcollector structure 28 which is sealed in a vacuum-tight manner to thecollector pole piece 23 via a cylindrical insulator 29. The beamcollector 28 is provided with water coolant channels, not shown, fordissipating the heat generated in the collector 28, in the conventionalmanner.

In use, the electron gun assembly is assembled with the cathode emitter3 disposed within a composite vacuum envelope structure defined by theenvelope of the gun, pump 13, valve 19, and beam collector 28. Asuitable oven is placed around the assembled components and the vacuumenvelope, including the gun, is baked at 450 to outgas the variouscomponents within the vacuum system. During the baking cycle, the systemis evacuated to a relatively low pressure by means of vacuum pump 13 andthe thermionic cathode emitter 3 is processed in the conventional mannerto activate the cathode emitter. During the bakeout cycle, thestraight-through valve 19 is in the open position to allow gascommunication throughout the entire vacuum envelope. After completion ofthe bakeout and cathode processing steps, the magnet 17 is removed fromthe pump 13 and the beam focus solenoid is placed in position asindicated in Fit]. 1 to produce a beamfocusing magnetic field throughthe beam passageway between the gun 2 and the collector 28. Thebeam-focusing magnetic field also supplies the magnetic field for thegetter ion vacuum pump 13. Beam voltage is applied to the gun and thegun 2 is put into operation for testing the beam. The beam current ismeasured via conventional techniques, i.e., by measuring the currentcollected by the collector to assure that the gun 2 is operatingproperly and that it meets design specifications.

Upon completion of the gun-testing step, the straightthrough vacuumvalve 19 is closed, thereby valving off the electron gun 2, pump 13, andthe closed-off portion of the valve 19. The gun 2, pump 13 and valve 19are then detached from the remaining portion of the vacuum envelopestructure by disassembling the demountable flange 24. The electron gun 2with the dependent pump and valve 19 are then removed from the guntester 1 for storage and subsequent use in a second evacuated system,such as a particle accelerator. Periodically, the vacuum pressure withinthe detached gun can be monitored by placing magnet 17 around the pump13 and energizing the vacuum pump. The current drawn by the pump is ameasure of the gas pressure within the gun and can be utilized tomaintain the pressure within the gun at a very low pressure, as of IOtorr.

Referring now to FIG. 2, there is shown a microwave linear particleaccelerator structure 31. The accelerator 31 includes a first bunchersection 32 comprising a disc-loaded waveguide, of conventional design,excited with microwave energy derived from a klystron oscillator 33 viaa waveguide 34. The electron gun 2, pump 13 and valve 19 are assembledto the buncher 32 via a demountable flange 24. A series of acceleratorsections 35, 36 and 37 are serially disposed along the beam path of theaccelerator and are excited via suitable sources of microwave energysuch as klystrons 38, 39 and 41 via waveguides 42,43 and 44,respectively. The accelerator structure including the buncher andaccelerator sections are evacuated to a suitable low pressure as of torrvia a conventional high-vacuum pump such as a diffusion pump, not shown.

When the accelerator structure has been evacuated, the straight-throughvalve 19 is opened such that the electron gun 2, in operation, canproject its beam axially through the pump 13, valve 19 and into thebuncher section 32 for acceleration by the accelerator section torelatively high particle velocities. The high-velocity beam is projectedthrough a suitable particle permeable window structure 45 disposed atthe output end of the accelerator. A beam focus solenoid structure 46surrounds the accelerator for focusing the beam therethrough andincludes a pole piece structure 47 at the gun end of the tube which isfitted to the anode pole piece 11, in the manner as indicated in FIG. 1.The beam focus solenoid 46 also pro vides the axial magnetic field forthe vacuum pump 13 which may be connected to a suitable power source formaintaining and monitoring the vacuum within the gun 2 and accelerator31. When the accelerator 31 is to be shut down or to stand forsubstantial periods without operation, the high-vacuum straight-throughvalve 19 is preferably closed to prevent contamination of the thermioniccathode emitter 3 in the event a leak develops in the acceleratorsection or in case work is done on the accelerator. As an alternative tosolenoid 46 for focusing the beam of the accelerator periodic magneticor electrostatic lenses may be employed.

The advantage of the processed, evacuated and demountable electron gunassembly is that the gun may be tested to assure that it meets designspecifications and then these specifcations can be preserved while thegun is in storage prior to its being affixed to a suitable utilizationdevice such as a linear accelerator. This offers many advantages since,prior to this time, the electron guns were assembled in a preprocessedstate onto the accelerator and were processed in place on theaccelerator structure. Leaks often developed in the accelerator andcontaminants such as oil from an oil-diffusion pump often contaminatedthe cathode 3 poisoning same such that it would not meet designspecifications. in such a case, it was difficult to tell whether thecathode failed because it was defective when shipped or because it wascontaminated in the processing stage when attached to the utilizationdevice. Use of the preprocessed, evacuated and demountable electron gungreatly reduces the failure of the guns when applied to variousutilization devices.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What I claim is:

1. In a method for manufacture and preservation of an electron gunassembly the steps of, assembling an electron gun having a thermioniccathode emitter in an envelope structure, evacuating the envelope,processing the cathode to activate same, applying beam voltage to thegun to draw beam current from the gun to determine proper operationthereof, valving off a portion of the envelope containing the electrongun by means of a valve in a portion of the envelope, and detaching thevalved-off and still evacuated portion of the envelope which containsthe valve and processed electron gun from the remainder of the vacuumenvelope structure for preserving the electron gun in an operablecondition for subsequent use in a second vacuum envelope structure.

2. The method of claim 1 including the step of, connecting a glowdischarge getter ion vacuum pump into that portion of the vacuumenvelope structure to be detached, and periodically activating the pumpto monitor the gas pressure within the detached and valved-offelectrongun.

3. The method ofclaim 1 including the step of attaching the evacuatedand processed electron gun to the vacuum envelope structure of aparticle accelerator, evacuating the particle accelerator envelopestructure, and opening the valve in the attached envelope portioncontaining the electron gun for placing the processed electron gun ingas communication with the remaining portion of the vacuum envelope ofthe particle accelerator.

4. The method ofclaim 1 wherein the step ofprocessing the electron gunto activate the cathode emitter includes the step of baking the electrongun and valve at a temperature in excess of 400C.

1. In a method for manufacture and preservation of an electron gunassembly the steps of, assembling an electron gun having a thermioniccathode emitter in an envelope structure, evacuating the envelope,processing the cathode to activate same, applying beam voltage to thegun to draw beam current from the gun to determine proper operationthereof, valving off a portion of the envelope containing the electrongun by means of a valve in a portion of the envelope, and detaching thevalved-off and still evacuated portion of the envelope which containsthe valve and processed electron gun from the remainder of the vacuumenvelope structure for preserving the electron gun in an operablecondition for subsequent use in a second vacuum envelope structure. 2.The method of claim 1 including the step of, connecting a glow dischargegetter ion vacuum pump into that portion of the vacuum envelopestructure to be detached, and periodically activating the pump tomonitor the gas pressure within the detached and valved-off electrongun.
 3. The method of claim 1 including the step of attaching theevacuated and processed electron gun to the vacuum envelope structure ofa particle accelerator, evacuating the particle accelerator envelopestructure, and opening the valve in the attached envelope portioncontaining the electron gun for placing the processed eleCtron gun ingas communication with the remaining portion of the vacuum envelope ofthe particle accelerator.
 4. The method of claim 1 wherein the step ofprocessing the electron gun to activate the cathode emitter includes thestep of baking the electron gun and valve at a temperature in excess of400*C.